The invention relates to a method of monitoring the filling level of a ball mill which is fed with coal to be pulverized and which includes a drum mounted to rotate on two bearings which are relatively far apart. A ball mill of this kind with a drum of cylindrical, biconical, or other shape is used in particular to feed pulverized coal to the burners of a coal-fired boiler, for example.
The level to which the ball mill is filled with coal must be kept substantially constant at all times to prevent excessive wear of the balls and for optimum transport of pulverized coal to the burners.
There are many methods of monitoring the filling level of a ball mill. A first method is based on measuring variations in the power absorbed by the electric motor driving rotation of the drum of the ball mill. A second method is based on measuring the noise level emitted by the ball mill in operation. A third method is based on the use of pneumatic sensors introduced into the interior of the drum of the ball mill. Finally, other methods are based on the use of gamma ray probes disposed inside the drum of the ball mill to detect the top and bottom levels of the layer of coal inside the drum.
As a general rule, the measurements employed in the above methods depend on the quality of the coal to be pulverized and in particular on its range of particle sizes and its moisture content. They also depend on the wear of the balls. Consequently, they are not always reliable.
The object of the invention is to propose a method of monitoring the filling level of a ball mill by using a reliable direct physical measurement which is independent of the quality of the coal to be pulverized, and in particular independent of its moisture content and its range of particle sizes. Another object of the invention is to propose a monitoring method which automatically takes account of the wear of the balls when the ball mill is operating and of replacement of the balls in the ball mill.
Document U.S. Pat. No. 3,960,330 describes a method in which the weight of the pulverizing system is measured using weight sensors for weighing the drum plus the support bearings of the drive gear for driving rotation of the drum, and said weight measured by the sensors is compared with a predefined set point value to regulate the feeding of the system with material to be pulverized.
The object of the present invention is to improve such a method and it provides a method of monitoring the level of filling of a ball mill which is fed with material to be pulverized and is provided with a drum mounted to rotate on two bearings which are relatively far apart, said method consisting in measuring the weight of the drum using weight sensors under the bearings supporting the drum of the ball mill and comparing the measured weight to a predefined set point value to regulate the feeding of material to be pulverized to the ball mill, the vertical component of the force created by the torque driving rotation of the drum being taken into account, the method being characterized in that said weight sensors are strain gauge weight sensors, and in that said vertical component is taken into account in the form of a correction of the weight measured by said sensors, performed before the comparison step, by means of a first weight value representative of said vertical component and obtained from a measurement of the power of the motor driving rotation of the drum.
The weight is a direct physical measurement of the level to which the ball mill is filled with coal and it is not influenced by the moisture content or the range of particle sizes of the load consisting of the mixture of coal and balls. The monitoring method of the invention is therefore very reliable. Also, the weight as measured in this way can easily be corrected by a computer program to allow for the vertical component of the torque driving the drum in rotation, the wear of the balls, as it varies with time, and the replacement of the balls in the ball mill. As a result, the method of the invention enables the level to which a ball mill is filled with coal to be monitored very accurately.